act 7:54

"When they heard this, they were furious and gnashed their teeth at him."

-Acts 7:54

The men of the Sanhedrin were listening to Stephen speak on how the Sanhedrin has rejected the Holy Spirit, the prophets who came telling of the Coming one, and Jesus Christ Himself. So they gnashed their teeth at Stephen. In other words they were grinding their teeth at him in anger. Often times this is the second stage of anger. First the eyebrows drop, second the face becomes hard, and your teeth begin to grind in anger. 

I all to often can relate with the Sanhedrin, my anger is not of God nor is my pride, it is something i struggle with. "When my heart was grieved and my spirit embittered, I was senseless and ignorant; i was a brute beast before you." (Psalm 73:21-22). I need to spend more alone time with God so he can free me from gnashing of teeth.